Hi David,
Most forms of sports photography present their own unique challenges, and equestrian is no different.
Usually the aim when photographing a horse and rider is to try to capture them at the peak of a jump, and usually the preferred angle is from in front of the rider as opposed to the rear. What I do like about this particular angle though is that you do get to appreciate the full stretch of the horse.
The one disappointment with this photograph though is that it is not sharp. Part of the problem is that you have used a rather large aperture of f5.6. A setting of f8 would have given you more depth of field.
Another problem is the slightly slow shutter speed of 1/640s, which has failed to stop this fast-moving subject. If you look closely at the front legs of the horse, particularly on the guards protecting the forelock you will notice that the edge of the protector is sharp but there is motion blur apparent at either end. Taking the shutter speed up to 1/1000s might have helped here.
Worth noting is that you cannot just close down the aperture and speed up the shutter speed (both of which will reduce the light into the camera) without adjusting the ISO. This image was made at 500 ISO and so in this instance you would need to up the ISO to about 1600 get the exposure accurate again.
Now, there are a few thicgs you could do in post-production to sharpen this image. In Photoshop go to Filters > Sharpen > Smart Sharpen and set the Amount to 100% and Radius to 5.0 Pixels. Choose Remove: Motion Blur and set the angle to 35°. This action will sharpen the entire image along the direction of movement.
As mentioned at the start… photographing horses (or any sport for that matter) does take practice. Ideally you want to try and get your focusing and panning techniques working well for you at the time you make the picture. You'll get better results and eliminate the need for corrective sharpening later on.
Cheers, Anthony